1. Field of the Invention
The present invention relates to a lighting control system for providing single or multicoloured illumination using microprocessor controlled, pulsed amplitude modulated (PAM) light emitting sources such as Light Emitting Diodes (LED). More particularly, the present invention is a method and apparatus for providing illumination control and which may for example optionally include temperature compensation to maintain light output and increase operating lifetimes throughout a wide range of operating temperatures.
2. Description of the Invention
Conventional illumination systems have been used for many years in general lighting and in lighting for decoration, advertising, warning, guidance and entertainment applications. Such illumination systems utilise a variety of light sources, including but not limited to incandescent, Halogen and Fluorescent types, which are subject to many drawbacks. For example, halogen and incandescent light sources produce undesirable heat and are limited to producing only white or yellow light. Therefore, in order to produce colours at different wavelengths, lighting systems based on these light sources may require substantial lens and filtering systems, which reduces desirability and increases costs significantly. These conventional sources may also have limited longevity with lifetimes significantly less than a few thousand hours. Such sources are also susceptible to breakage in high shock and vibration prone environments.
Light Emitting Diode (LED) sources have recently undergone significant advances, which enables them to be a cost effective replacement for conventional light sources. LED light sources offer significant benefits over conventional light sources as they consume less electrical energy for a given light intensity while exhibiting much longer lifetimes. Other desirable properties of LEDs include high resistance to shock or vibration, low heat dissipation, very fast switching response times and a wide choice of illuminating colours.
However, there are different issues to consider when using LEDs where conventional lamps have historically been used. Primarily, LEDs are heat-sensitive devices and, therefore, heat generated during operation of the LEDs and associated components must be dissipated adequately to ensure a reliable operation over extreme environmental temperature ranges. The life expectancy of an LED lighting system would be significantly reduced or complete failure could occur if environmental conditions were not used as a feedback control to reduce the operating parameters of the LEDs within such a system. Also, the majority of LED light sources have limited illumination spectra about a particular wavelength making it difficult to produce a wide colour spectrum that responds to the characteristics of the human eye. Many such LED based lighting systems have been proposed to enable colour mixing using three primary colours usually with illumination spectrums at wavelengths corresponding to Red (605 to 635 nm), Green (530 to 570 nm) and Blue (455 to 490 nm). However, these systems have a poor colour rendering index compared to conventional light sources and therefore objects illuminated from these systems do not appear pleasing to the human eye. Many of these systems use a Pulse Width Modulation (PWM) technique which operates a current controlled current source to control the current through the LED devices however, systems based on the PWM technique are more likely to generate radio frequency interference (RFI) and careful design criteria is required to reduce RFI, increasing the complexity and the number of components within such systems. Variations in the pulse width of the output used to drive the LED loads are also unacceptable in, for example, high-speed inspection systems which utilise solid state camera systems such as Charge Coupled Device (CCD) or Complementary Metal Oxide Semiconductor (CMOS) image sensors.